The US space agency is assessing a problem with one of two cooling systems aboard the International Space Station.

Nasa officials say the situation is potentially serious but not life-threatening.

The system automatically shut itself down after detecting abnormal temperatures, said a spokesman.

The two external cooling loops circulate ammonia outside the station to keep both internal and external equipment cool.

Nasa told NBC News that the situation might require the station crew to perform an emergency spacewalk.

"We are cleared for a contingency spacewalk if we need to do one," spokesperson Kelly Humphries told the channel.

In a statement, Nasa said flight control teams had been working to get the cooling loop back up and running.

They suspect a malfunction has affected a flow control valve inside the pump itself.

In the meantime, the teams have moved certain electrical systems over to the second loop, but the space agency stressed that the crew and the station was in no danger.

Some non-critical systems have also been powered down inside the Harmony node, the Kibo laboratory and the Columbus laboratory whilst engineers work to figure out what caused the problem and how to fix it.

The statement said: "The crew is safe and preparing to begin a normal sleep shift while experts on the ground collect more data and consider what troubleshooting activities may be necessary."